1. Field of the Invention
The present invention relates specifically to a frequency modulation apparatus, polar modulation transmission apparatus, wireless transmission apparatus, and wireless communication apparatus that perform frequency modulation by utilizing the PLL (Phase Locked Loop).
2. Description of the Related Art
Heretofore, frequency modulation apparatuses utilizing the PLL have been widely used to modulate carrier signals with baseband modulation signals and form transmission signals (that is, to up-convert baseband modulation signals to radio frequency). Generally, frequency modulation apparatuses of this type are expected to realize low costs, low power consumption, excellent noise characteristics, and high modulation accuracy. To modulate signals using the PLL, and, in particular, to improve modulation accuracy using the PLL, the PLL frequency bandwidth (i.e. PLL bandwidth) is preferably wider than the frequency bandwidth of the modulation signal (i.e. modulation bandwidth).
However, widening the PLL bandwidth has the risk of deteriorating noise characteristics. So, the technology called “two-point modulation” is presently proposed whereby the PLL bandwidth is set narrower than the modulation bandwidth and the modulation in the PLL bandwidth and the modulation outside the PLL bandwidth are performed at two different points (see, for example, U.S. Pat. No. 4,308,508).
FIG. 1 shows the configuration of a frequency modulation apparatus utilizing a conventional two-point modulation PLL. Frequency modulation apparatus 10 has: a voltage controlled oscillator (VCO) 1 that changes the oscillation frequency in accordance with the voltage in the control voltage terminal; frequency divider 2 that divides the frequency of an RF modulation signal outputted from VCO1; phase comparator 3 that compares the phase of the output signal of frequency divider 2 and the phase of the reference signal and outputs a signal in accordance with the phase difference between the two signals; and loop filter 4 that averages the output signals of phase comparator 3 and outputs the result. Frequency modulation apparatus 10 adds modulation data generated in modulation signal generator 5 to carrier frequency data and supplies the result as the frequency division ratio in frequency divider 2, thereby performing modulation at the first point.
In addition, frequency modulation apparatus 10 is designed to convert the modulation data into analogue voltage in digital-to-analogue (D/A) convertor 6, suppress the aliasing noise in the output of D/A convertor 6 by means of post filter 7 and thereafter add the result to the output of loop filter 4 and supply this result to the control voltage terminal of VCO 1, thereby performing modulation at the second point.
Thus, the use of the two-point frequency modulation technology such as described above makes it possible to output wideband RF modulation signals that stretch outside the PLL bandwidth, even when the PLL bandwidth is set narrower than the modulation bandwidth. As a result, the deterioration of noise characteristics due to the PLL is suppressed.
First, assuming a case using two-point modulation, frequency characteristics will be described below. FIG. 2 shows frequency characteristics in baseband area for explanation of the operation of a two-point modulation PLL. In this drawing, H(s) is a transfer function that indicates the frequency characteristics of the PLL, where s=jω. H(s) has low pass characteristics, such as shown in FIG. 2. The modulation signal added to the frequency division ratio set in frequency divider 2 is low pass filtered by the transfer function H(s) by the PLL. On the other hand, the modulation signal outputted from post filter 7 is added to the control voltage terminal of VCO 1 and thereby high pass filtered by the transfer function 1−H(s), such as shown in FIG. 2. That is, if the modulation data is Φ(s), the baseband component in the RF modulation signal outputted from VCO 1 bears no relationship to the frequency characteristics of the PLL as represented by the following formula:H(s)Φ(s)+{1−H(s)}Φ(s)=Φ(s)  (1)
Applying two-point modulation thus to the PLL makes it possible to output wideband RF modulation signals that stretch outside the PLL bandwidth, from VCO 1. Incidentally, fs is the sampling frequency (i.e. the operation frequency of D/A convertor 6).
Now, the output of D/A convertor 6 contains quantization noise such as shown in FIG. 2. suppressing this quantization noise is required from post filter 7. Post filter 7 has low pass characteristics, such as shown in FIG. 2. When the bandwidth is too narrow, this suppresses the modulation signal shown in FIG. 2. On the other hand, although widening the bandwidth too much makes it possible to avoid adversely affecting the modulation signal, this in turn makes it difficult to suppress noise enough including quantization noise.
FIG. 3 illustrates the spectrum of RF modulation signal that appears in the output of VCO 1. Quantization noise is superimposed upon the modulation signal and the resulting waveform is outputted from VCO 1. Quantization noise is suppressed near the center frequency (fVCO), by virtue of the characteristics of 1−H(s) shown in FIG. 2. Likewise, quantization noise is suppressed far from fVCO, by virtue of the frequency characteristics of post filter 7.
Now, for example, the GSM (Global System for Mobile Communications) standard stipulates that the spectrum of transmission waves is not to be out of the spectrum mask such as shown by the dotted line in the drawing.
Nevertheless, according to the conventional two-point frequency modulation apparatus, the spectrum of transmission waves might go beyond the spectrum mask due to the influence of quantization noise, such as shown in FIG. 3.